The solid electrolyte cells of the kind using .beta.-alumina as the electrolyte have recently been proposed for use, because of their long shelf life, higher generating voltage per small area and relative freedom from leakage of the internal solvent. However such electrolyte cells have not been commercialized as they do not have sufficient efficiency at the room temperature.
Compounds having water of crystallization have been proposed for the cathodic active material to provide a stable discharge voltage. However, such cells exhibit a defect that the discharge voltage falls rapidly at low temperatures ranging from 0.degree. C. to -20.degree. C.